examples: add FindMy example showing how to create an AirTag style BL…

…E beacon.

The data format is be compatible with OpenHaystack and other similar ways to
use the Apple FindMy network to track personal Bluetooth devices.

Signed-off-by: deadprogram <[email protected]>

examples/findmy/main.go

@@ -0,0 +1,84 @@
+// example showing how to use the Bluetooth package to advertise a FindMy compatible device.
+// aka AirTag
+// see https://github.com/biemster/FindMy
+//
+// To build:
+// tinygo flash -target nano-rp2040 -ldflags="-X main.PublicKey='SGVsbG8sIFdvcmxkIQ=='" ./examples/findmy
+//
+// For Linux:
+// go run ./examples/findmy SGVsbG8sIFdvcmxkIQ==
+package main
+
+import (
+	"encoding/base64"
+	"errors"
+	"time"
+
+	"tinygo.org/x/bluetooth"
+)
+
+var adapter = bluetooth.DefaultAdapter
+
+func main() {
+	time.Sleep(2 * time.Second) // wait for USB serial to be available
+
+	key, err := getKeyData()
+	if err != nil {
+		panic("failed to get key data: " + err.Error())
+	}
+	println("key is", PublicKey, "(", len(key), "bytes)")
+
+	opts := bluetooth.AdvertisementOptions{
+		AdvertisementType: bluetooth.AdvertisingTypeNonConnInd,
+		Interval:          bluetooth.NewDuration(1285000 * time.Microsecond), // 1285ms
+		ManufacturerData:  []bluetooth.ManufacturerDataElement{findMyData(key)},
+	}
+
+	must("enable BLE stack", adapter.Enable())
+	adapter.SetRandomAddress(bluetooth.MAC{key[5], key[4], key[3], key[2], key[1], key[0] | 0xC0})
+	adv := adapter.DefaultAdvertisement()
+
+	println("advertising...")
+	must("config adv", adv.Configure(opts))
+
+	println("starting...")
+	must("start adv", adv.Start())
+
+	address, _ := adapter.Address()
+	for {
+		println("FindMy /", address.MAC.String())
+		time.Sleep(time.Second)
+	}
+}
+
+func getKeyData() ([]byte, error) {
+	val, err := base64.StdEncoding.DecodeString(PublicKey)
+	if err != nil {
+		return nil, err
+	}
+	if len(val) != 28 {
+		return nil, errors.New("public key must be 28 bytes long")
+	}
+
+	return val, nil
+}
+
+func findMyData(keyData []byte) bluetooth.ManufacturerDataElement {
+	data := make([]byte, 0, 27)
+	data = append(data, 0x12, 0x19)        // Offline Finding type and length
+	data = append(data, 0x10)              // state
+	data = append(data, keyData[6:]...)    // copy last 22 bytes of public key
+	data = append(data, (keyData[0] >> 6)) // First two bits
+	data = append(data, 0x00)              // Hint (0x00)
+
+	return bluetooth.ManufacturerDataElement{
+		CompanyID: 0x004C, // Apple, Inc.
+		Data:      data,
+	}
+}
+
+func must(action string, err error) {
+	if err != nil {
+		panic("failed to " + action + ": " + err.Error())
+	}
+}

examples/findmy/mcu.go

@@ -0,0 +1,6 @@
+//go:build tinygo
+
+package main
+
+// PublicKey is the public key of the device. Must be base64 encoded.
+var PublicKey string

examples/findmy/os.go

@@ -0,0 +1,8 @@
+//go:build !tinygo
+
+package main
+
+import "os"
+
+// PublicKey is the public key of the device. Must be base64 encoded.
+var PublicKey = os.Args[1]